In automotive vehicles, side sills are provided to extend along the opposite sides of a floor panel of a passenger compartment in a front-rear direction of the vehicle body, and center and other pillars are fixed at their lower end portions to the side sills. In recent years, there has been developed a vehicular side body structure where a side sill reinforcing member formed by extruding an aluminum alloy is disposed inside a side sill body of a closed cross-sectional shape formed of a steel plate, in order to reduce the overall weight of the side sill and increase the rigidity of the side sill.
FIGS. 7A and 7B hereof are vertical sectional views showing the vehicular side body structure. As shown in FIG. 7A, the side sill 101 comprises an inner side sill member 102 formed of a steel plate and disposed adjacent to the passenger compartment, an outer side sill member 103 formed of a steel plate and disposed outwardly of (i.e., closer to the outside of the vehicle than) the inner side sill member 102, and a side sill reinforcing member 104 formed by extruding an aluminum alloy and disposed inside the inner side sill member 102 and outer side sill member 103. These inner and outer sill members 102 and 103 and side sill reinforcing member 104 are fixedly joined together at their respective first flanges 102a, 103a and 104a and at their respective second flanges 102b, 103b and 104b. 
Center pillar 106 comprises an inner center pillar member 107 disposed adjacent to the passenger compartment, and an outer center pillar member 108 disposed outwardly of the inner center pillar member 107. The inner center pillar member 107 is fixed at its lower end portion to the flange 102a, and the outer center pillar member 108 is fixed at its lower end portion to a side surface of the outer side sill member 103. Reference numeral 109 indicates a floor panel fixed to a side surface of the inner side sill member 102.
As shown in FIG. 7B, if a great external force acts on the center pillar 106 from the side, the center pillar 106 would be deformed to incline as indicated by an arrow, due to which the external force (torsional force) transmits to the inner and outer side sill members 102 and 103 and thus the inner and outer side sill members 102 and 103 would be deformed to incline toward the passenger compartment.
Because the inner side sill member 102, outer side sill member 103 and side sill reinforcing member 104 are fixedly joined together only at their respective first flanges 102a, 103a and 104a and at their respective second flanges 102b, 103b and 104b as noted above, the side sill reinforcing member 104 hardly follows the inclining deformation, due to the external force, of the inner and outer side sill members 102 and 103. Namely, the side sill 101 can not appropriately retain its sectional shape during input thereto of the external force. As a consequence, the side sill reinforcing member 104 can not achieve a sufficient reinforcement effect; in other words, the side sill 101 tends to have a low rigidity against torsional force.
Further, French Patent Application Laid-Open Publication No. FR 2890361 A1 discloses another vehicular side body structure where a reinforcing member formed by extruding an aluminum alloy is provided inside a side sill. FIG. 8 is a sectional view of the vehicular side body structure disclosed in FR 2890361 A1. In this vehicular side body structure, the side sill 112 comprises an inner panel 111 and an outer panel 112 fixed to the outer end of the inner panel 111, and the reinforcing member 117 formed by extruding an aluminum alloy and having a generally inverted-S sectional shape is disposed inside the side sill 113 and fixed to the inner panel 111 by means of fastener members 114 and 116.
Center pillar is fixed at its lower end portion to the inner and outer panels 111 and 112. If an external object collides against the center pillar from the side, a torsional force would be applied to the side sill 113 due to deformation, toward the passenger compartment, of the center pillar. However, because the reinforcing member 117 of the generally inverted-S sectional shape tends to easily deform at its individual end portions and thus does not have a (sufficient rigidity against the torsional force, it is difficult for the reinforcing member 117 to produce a sufficiently great reactive force to the torsional force. Also, because the reinforcing member 117 does not have a great rigidity against a bending force acting on the side sill 113 from the side, it is difficult for the reinforcing member 117 to produce a sufficiently great reactive force to the bending force.